1. Field of the Invention
The invention relates generally to subsea dispersant systems and methods. More particularly, the invention relates to dispersant systems and methods for managing the subsea release or venting of hydrocarbons.
2. Background of the Technology
In offshore drilling and production operations, hydrocarbons may be discharged or vented into the surrounding sea water. The subsea release of hydrocarbons may present environmental issues. In addition, the subsea release of hydrocarbons may potentially present a hazardous environment at the surface.
Chemical dispersing agents, or simply dispersants, are specially formulated chemical products containing surface-active agents and a solvent. Dispersants aid in breaking up hydrocarbon solids and liquids by reducing the interfacial tension between the oil and water, thereby promoting the migration of finely dispersed water-soluble micelles that are rapidly diluted. As a result, the hydrocarbons are effectively spread throughout a larger volume of water, and the environmental impact may be reduced. In addition, dispersants are believed to facilitate and accelerate the digestion of hydrocarbons by microbes, protozoa, nematodes, and bacteria. Moreover, the use of dispersants reduces the risk to responders at the surface by minimizing the accumulation of oil, associated volatile organic compounds (VOCs) and hydrocarbon vapors. Dispersants can also delay the formation of persistent oil-in-water emulsions.
Traditionally, dispersants have been sprayed onto the oil at the surface of the water. Normally, this process is controlled and delivered from surface vessels or from the air immediately above the oil at the surface. For example, aircraft may be employed to spray oil dispersant over an oil slick on the surface of the sea. For some types of chemical dispersants, the composition of the dispersant itself may present an additional environmental concern. Thus, minimizing the quantity and distribution of dispersants is generally preferred. However, since oil released from a subsea well diffuses and spreads out at it rises to the surface, oil at the surface is often spread out over a relatively large area (e.g., hundreds or thousands of square miles). To sufficiently cover all or substantially all of the oil that reaches the surface, relatively large quantities of dispersant must be distributed over the relatively large area encompassed by the oil slick.
To minimize “overspray” and limit the application of dispersants to the oil slick itself, distribution at the surface typically involves the visualization of the oil slick at the surface. Accordingly, around the clock surface distribution may not be possible (e.g., at night the location and boundaries of the oil slick at the surface may not be visible). However, there is usually a limited time-frame in which dispersants can be successfully applied at the surface. In particular, certain oil constituents evaporate quickly at the surface, leaving waxy residues or “weathered” oil that are often unresponsive to dispersants.
It should also be appreciated that some turbulence at the surface (e.g., wave action) is preferred during surface application of dispersants to sufficiently mix the dispersant into the oil and the treated oil into the water. Depending on the weather and sea conditions, surface turbulence may be less than adequate. Moreover, by limiting distribution of dispersants to the surface, only those microbes at or proximal the surface have an opportunity to begin digestion of the oil.
Accordingly, there remains a need in the art for improved systems and methods for the offshore application of chemical dispersant to discharged hydrocarbons. Such systems and methods would be particularly well received if they offered the potential to minimize the quantity of dispersants emitted, enhance dissipation of the discharged oil before it reaches the surface, operate around the clock (e.g., 24 hours a day), and facilitate increased microbial digestion of oil.